The present invention relates to magnetic thin film media for longitudinal recording, and more particularly to protective overlayers applied to the magnetic thin films of such media.
Magnetic recording involves magnetic disks typically housed within disk drives and rotated on an axis, with data transducing heads positioned at close proximity to the recording surfaces of the disks and moved generally radially with respect to the disks. Generally the disks are of two kinds: flexible or "floppy" disks in contact with associated transducing heads at all times; and rigid disks. Rigid disks are rotated at much higher speeds than the flexible disks. Consequently the transducing heads, during reading and recording operations, are maintained at a controlled distance from the disk recording surfaces, supported on a "bearing" of air as the disks rotate. Such transducing heads contact their associated rigid disks only when the disks are stationary, when they accelerate from a stop, or when they decelerate just before coming to a complete stop.
As compared to flexible disks, rigid disks and drives are subject to stricter design tolerances, arising from the greater density of data stored on disk recording surfaces. It is considered desirable during reading and recording operations to maintain each transducing head as close to its associated recording surface as possible, i.e. to minimize the "flying" height of the head. A reduction in flying height improves signal to noise ratio, enhancing the speed and accuracy of reading and recording operations, and increasing the density at which data can be stored.
The utility of protective cover layers in magnetic recording media is well known. Materials employed as magnetic thin films, typically magnetic alloys such as cobalt-nickel, cobalt-chromium-tantalum, cobalt-chromium-platinum, cobalt-nickel-platinum, cobalt-nickel-chromium, and cobalt chromium among others, are susceptible to corrosion, particularly in relatively humid environments. Moisture in the air reacts with exposed surface areas of the media, degrading or destroying the capability of the material to be magnetized at corrosion sites. This diminishes the capacity of the magnetic medium to store data, and gives rise to the desire to protect the magnetic thin film against exposure to moisture. While corrosion protection of the magnetic alloy is required of the overcoat, the tribological or wear properties of the overcoat are of primary concern. Even in the case of rigid disks, there is a substantial amount of surface contact between the disks and transducing heads. The overcoat must maintain its surface integrity and interact with the transducing head in a manner to minimize wear or damage to the transducing head.
In connection with rigid disk media, the traditional approach to meeting this need is to apply a film or overcoat of a hard, amorphous carbon. Carbon coatings, however, tend to develop pin holes that expose the magnetic recording film to corrosion. Particulates formed at corrosion sites can interfere with the reading and writing of data, and can eventually cause a data transducing head to crash. Corrosion sites frequently are slightly raised, and cause aerodynamic instability in the air bearing which supports the transducing head relative to its associated recording surface. Resulting fluctuations in the head can lead to a head crash, particularly at low flying heights. Carbon films also are susceptible to local microscopic heating due to head contact friction. Locally intense heat converts carbon from the SP-3 phase to the SP-2 phase, a transformation that can lead to sublimation of the carbon overcoat, and subsequently increased wear.
Recent designs for rigid disk drives also contemplate disk rotation at speeds up to 5,000 revolutions per minute, as compared to the 1,800 rpm in conventional drives. The result is a need for improved tribological qualities in the protective overcoat, due to the higher speeds themselves, and also due to the fact that higher disk rotational speeds result in the transducing head contacting the disk, particularly the overcoat, for longer periods of time. Carbon overcoats wear rapidly at speeds approaching 5,000 rpm, due to such increased contact with the transducing head during acceleration and deceleration of the disk. Such wear eventually can expose the underlying magnetic layer.
Frequently, liquid lubricants have been applied to improve tribological characteristics of carbon and silicon coatings, and do improve both friction and wear characteristics. Lubricant layers are increasingly subject to wear or spin-off at higher rotational speeds, eventually exposing the carbon layer beneath the lubricant. Chromium underlayers have been applied immediately beneath carbon coatings in an attempt to resist corrosion of the carbon layers. While this technique has successfully increased resistance to corrosion, the combined layer thickness reduces signal amplitude to the head.
In view of the deficiencies in carbon overcoats, alternative cover layer materials have been explored. For example, European Patent Application No. 87200616.8 (Yamashita) discloses a variety of protective coatings based on zirconium, including zirconium oxide (zirconia), zirconium boride, zirconium carbide and zirconium nitride. The coatings are deposited by sputtering, and preferably include a stabilizer such as yttrium oxide (Y.sub.2 O.sub.3), calcium oxide or magnesium oxide. Yamashita teaches that an intermediate layer enhances control of the reliability of the magnetic disk, stating that a zirconia film formed directly onto magnetic media may blister and peel due to stress at the bond between the cover layer and medium, caused by friction from contact with the transducing head. This failure was reported as observed after just one hundred CSS (contact start stop) test cycles. An intermediate layer of chromium, zirconium, hafnium, titanium, tantalum or tungsten, or an alloy of these materials, is said to reduce this problem. Of course, it also adds to the thickness of the material covering the magnetic thin film recording medium.
Therefore, it is an object of the present invention to provide a magnetic recording medium in which a zirconia or hafnia based cover layer is applied directly to a magnetic thin film recording layer, with minimal degradation of the cover layer tribology, even after thousands of CSS cycles.
Another object is to provide a process for manufacturing a magnetic recording medium, in which a protective overlayer is applied directly to a magnetic thin film layer in a reactive sputtering process favorable to forming zirconia or hafnia upon the thin film, thus to enhance the quality of the overcoat.
Yet another object is to provide a method of treating a magnetic thin film recording layer of a recording medium, just prior to forming a protective cover layer onto the magnetic film, to enhance adhesion of the cover layer to the film for improved performance and longer life of the resulting recording medium.